1. Field of Invention
This invention relates to holography, and more particularly to volume holography.
2. Description of the Prior Art
A hologram is a diffraction grating having bars corresponding to fringes of an interference pattern typically formed by an interaction of two coherent beams of light of a given wavelength, referred to as an object beam and a reference beam, respectively. The wavefront of the object beam is usually amplitude modulated by an object, such as a photographic transparency. The diffraction grating is a photograph of the interference pattern and is referred to as a holographic record of the transparency.
In a volume hologram, the beams interact within a recording medium at a depth that is much greater than spacing between adjacent fringes, thereby causing the depth within the medium of the diffraction grating to be much greater than the spacing between adjacent bars.
The medium typically has the shape of a rectangular parallelpiped whereby the medium has flat surfaces. Two of the surfaces that are opposite from each other are referred to as a recording surface and a read-out surface, respectively. During the recording of the volume hologram, the beams are transmitted to the recording surface. After the volume hologram is recorded, a reconstruction of the object beam emanates from the read-out surface in response to a reconstructing beam being transmitted to the recording surface. Well known optical apparatus is used to provide an image that corresponds to modulation of the reconstruction.
Unlike other types of holograms, the reconstruction emanates from the volume hologram only when the reconstructing beam is transmitted to the recording surface substantially over the path of transmission of the reference beam during the recording of the volume hologram. Additionally, the reconstruction and a normal to the read-out surface form an angle that is equal to the angle of incidence of the object beam to the recording surface during the recording of the volume hologram. It should be understood that the reconstructing beam is comprised of coherent light of the given wavelength.
When the reconstructing beam is transmitted to the recording surface over the path of transmission of the reference beam, a condition, known as the Bragg condition, is said to be satisfied. Therefore, the reconstruction emanates from the volume hologram only when the Bragg condition is satisfied. Because of the Bragg condition, the medium may be used for recording a multiplicity of volume holograms as explained hereinafter.
In recording a book, for example, the recording surface is initially exposed to cause a recording in the medium of a volume hologram of a photographic transparency of the first page of the book. After the recording of the first page, the medium is incrementally rotated (through an angle of 1.degree., for example) with respect to the reference and the object beams. Thereafter, the recording surface is exposed to cause a recording of a volume hologram of a photographic transparency of the second page of the book and the medium is again incrementally rotated. By alternately recording and incrementally rotating, as many as 180 photographic transparencies of pages of the book can be recorded in the medium to comprise a record of the book.
Since the recording of the photographic transparencies of the book may include 180 exposures and 179 rotations of the medium, the record is expensive. Additionally there is no known inexpensive method for providing a duplicate of the record.